This invention relates to a system and method for mounting integrated circuits onto printed circuit boards. This invention also relates to a method for testing integrated circuits and printed circuit boards.
Electronic circuit boards for computers and other devices typically consist of many integrated circuits mounted on a printed circuit board (PCB). Individual integrated circuit (IC) packages have a semiconductor chip or die enclosed within a protective plastic package. A lead frame consisting of multiple conductive leads provides the electrical interconnect between the enclosed semiconductor chip and components exterior to the plastic package.
The PCB has many conductive traces arranged on its surface according to a selected pattern for efficiently transferring electronic signals across the board to and from individual IC packages. The traces on the PCB have terminal locations or bonding sites which define the locations where specific IC packages are to be located. As the IC package is being mounted to the PCB, the conductive leads of the IC package are aligned with corresponding bonding sites on the PCB. The conductive leads are then soldered to the corresponding bonding sites to permanently mount the IC package to the PCB.
One significant problem in the mounting process is how to initially position the IC packages on the PCB and hold them there temporarily until the soldering step is performed. According to conventional techniques, a screen with multiple holes is positionally and carefully aligned with the PCB so that the screen holes are superimposed on the bonding sites on the PCB. Solder paste is then squirted through the holes of the screen onto the bonding sites of the PCB. Solder paste is a conductive material which forms a temporary glue.
After the solder paste has been applied to the bonding sites, the IC packages are positioned atop the PCB. The solder paste temporarily adheres the conductive leads of the IC package to the corresponding bonding sites. Solder is then applied to the bonding sites, and the PCB is subjected to ultraviolet light or other techniques for causing the solder to melt and completely surround and secure the conductive leads of the IC package to the bonding sites.
The use of solder paste for temporarily mounting IC packages onto PCBs creates numerous problems. First, the process of positioning and aligning the screen over the PCB and squirting the solder paste through the screen holes is a difficult, time-consuming task. Second, the solder paste is an extremely messy substance and can cause problems if not precisely applied. Third, after the permanent solder is applied, the PCBs must be cleaned with special cleaning materials, such as freon-type cleaners, to remove all of the solder paste. Accordingly, the present mounting technique introduces several additional manufacturing steps that reduce overall manufacturing efficiency while increasing costs.
The present invention eliminates the above-mentioned drawbacks by providing a system and method for mounting integrated circuits onto printed circuit boards without the use of solder paste.
Another drawback in conventional manufacturing techniques is that, when an IC package fails during the testing, it is often too difficult to break the solder bonds and remove the defective IC package without damaging the PCB or other components. Thus, the entire manufactured board is discarded, even though most of the board and IC packages are operative. This causes significant waste because otherwise operable components are needlessly thrown away.
The present invention provides a method for testing integrated circuits and printed circuit boards prior to the soldering step. This testing method thereby permits removal of defective components without discarding the operable components.